Low radiation open-boat crucibles

ABSTRACT

An open boat, stainless steel crucible having at its upper surface a shallow dish portion suitable for containing a charge, and an under surface containing a thin homogeneous outer layer of gold.

United States Patent Relyea Aug. 26, 1975 [54] LOW RADIATION OPEN-BOAT CRUCIBLES 2,195,436 4/1940 Weller 432/264 2,947,114 8/1960 Hill [75] Inventor: Relyea Webster 3,430,937 3/1969 Spitzer 432/264 [73] Assignee: Xerox Corporation, Stamford,

Conn' Primary ExaminerJohn J. Camby [22] Filed: Apr. 26, 1974 [21] Appl. No.: 464,581

[57] ABSTRACT i 3i An open boat, stainless steel crucible having at its [58] Field 01' Search 432/253, 264, 265; 220/64 "P Surface a Shanw sutable taming a charge, and an under surface containing a 5 6] References Cited thln homogeneous outer layer of gold.

UNlTED STATES PATENTS 8 Claims, 6 Drawing Figures 1,043,579 11/1912 Eldrcd 432/264 PATENTED Amzzsms 3 90 1 ,647

SHEET 1 UF 5 LOW RADIATION OPEN-BOAT CRUCIBLES BACKGROUND OF THE INVENTION The instant invention broadly relates to an evaporation crucible, and more specifically to a crucible suitable for use in the vacuum deposition of thin films.

Generally in the practice of evaporating relatively thin films of metals or nonmetals onto suitable substrates, the crucibles or evaporation vessel commonly employed comprises an open boat having a somewhat rectangular cross section.

The radiation emitted from the crucible contributes greatly to the heat change in the substrate to be coated. Where particularly thin substrates are to be coated, this radiant heat from the crucible may be particularly undesirable, especially if the substrate is to be maintained at a relatively low temperature or within a carefully controlled temperature range. In order to compensate for this radiation effect, various coating parameters are usually varied in order to reduce the radiation from the crucible. For example, the distance from the substrate to the crucible may be increased if the radiation is too great. Alternatively, the amount of evaporant load within the crucible may be increased or reduced, in order to control the rate of deposition and thus indirectly control the substrate temperature.

It can therefore be seen that if the crucible itself could be designed to emit relatively low radiation, that this alone will allow for a much greater variance in the coating parameters.

The above problems are particularly relevant to vacuum evaporation techniques which are used in forming photoconductive coatings on the surface of metallic belts, drums or plates which are used as the photosensitive member in xerography.

OBJECTS OF THE INVENTION It is therefore an object of this invention to provide an open boat crucible which exhibits relatively low radiation.

It is a further object of this invention to provide an open boat crucible in which the major portion of the radiation is emitted from the top side of the crucible which is charged with the evaporant material.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with this invention by providing a novel evaporation crucible which exhibits low radiation. Generally, open boat crucibles have a shallow dish portion at their upper surface which is adapted to hold a suitable charge of evaporant material. These open boat crucibles are generally rectangular in cross-section, and are usually provided with a downward extending plate at each end. The end plates are suitable for attaching the crucible to other crucibles in the form of an array or plurality of crucibles, and may also be connected to electrical contacts suitable for use in resistance heating.

In accordance with the present invention in order to reduce stray radiation from the back or bottom surface of the crucible, an infrared reflecting layer of gold is plated over the bottom surface only. The affect of this gold layer, is to reduce the infrared radiation coming through the back of the crucible during evaporation, and in effect directs the bulk of the radiation to the charge contained in the top of the crucible. Therefore, most of the radiant energy emitted from the crucible is effectively used to melt the charge in the crucible.

As will be more clearly demonstrated in greater detail later, the use of a polished gold backed crucible as compared to an unpolished crucible having no gold backing results in a reduction from about 200C to C in the temperature of the radiation emitted from the crucible backing, thus significantly reducing the infrared radiation emitted from the crucible during operation. The crucibles of the present invention are applicable to evaporating metals or nonmetals on any substrate for any suitable use, but have particular utility for the formation of photoconductive coatings for use in xerography.

A preferred use for these crucibles is in the deposition of selenium and selenium alloys onto supporting substrates such as metallic drums, plates or flexible substrates. A more specific application is in this use in which a plurality of substrates such as drums are arrayed circumferentially around a planar array of crucibles and the entire array of drums revolved around the crucible with each drum also rotating around its horizontal axis. This concept is more specifically defined in U.S. Pat. application Ser. No. 244,374, filed Apr. 17, 1972.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of the instant invention will become apparent upon consideration of the following disclosure of the invention, especially when taken in conjunction with the following drawings wherein:

FIGS. 1A and 1B illustrate a top and bottom view of an open boat crucible for the present invention.

FIG. 2 illustrates a side view of an array of crucibles.

FIG. 3 illustrates a cross-sectional view of one edge of a crucible contemplated for the present invention.

FIG. 4 illustrates a plot of the radiation temperature emitted from the bottom side of three different crucibles.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1A is a top view illustrating the general configuration of a crucible of the present invention. Reference character 10 illustrates an open boat crucible of the type suitable for use in the present invention. Crucible 10 has somewhat shallow cavity 11 adapted to hold an evaporant charge. The crucible further contains end plates 13 and 14 having a plurality of slots 15 adapted to be connected to a source of electrical power or for bolting a plurality of crucibles together. FIG. 1B illustrates a back or bottom view of crucible 10 having a back side 12.

FIG. 2 illustrates an array of crucibles 10 which are either welded or bolted together at ends 13 and 14. Generally, the plate, drum or other work piece to be coated is suspended above the crucible array. The crucibles are filled with the appropriate evaporant material and the array heated, usually by electrical resistance, with the evaporant vapors flowing upward and condensing on the substrate surface. If carried out under vacuum, the entire array of crucibles and substrates are contained within a vacuum chamber.

FIG. 3 illustrates one embodiment of the present invention and is a partial cross-sectional view through the body of one of the open boat crucibles. Reference character 20 represents the main body of the crucible and may comprise any suitable metal, such as tantalum, molybdenium, columbium or stainless steel. Stainless steel is particularly preferred in that it is relatively inexpensive, resistant to corrosion, and can readily be stamped or otherwise formed into crucibles of the type and shape used in the present invention. The upper surface, including the shallow dish portion or cavity which contains the evaporant charge, is preferably coated with a thin layer 21 of SiO in order to reduce oxidation and scaling of the upper surface of the crucible. The SiO layer is formed by vacuum evaporation using quartz as the source or evaporant material at a vacuum of about l- Torr. Thin coatings about 800-9,000 Angstrom Units have been found satisfactory. The bottom surface of the crucible is first coated with a protective inorganic coating 22, of any suitable silicate, such as potassium silicate, and then overcoated with a highly reflective thin layer of gold 23. The potassium silicate layer may be formed over the crucible backing by coating using an aqueous solution of distilled water containing about 20 percent by weight potassium silicate. The thickness of the potassium silicate layer may range from about 2,500 Angstrom Units to 1 micron (10,000 Angstrom Units). The gold layer may be formed by any suitable technique well known to the art such as by electrodeposition or by vacuum deposition. It is essential that the gold layer be thick enough to be infrared red radiation reflective. It has been found that this thickness for gold should therefore be at least about 600 Angstrom Units. A thickness of 800 Angstrom Units for the gold has been found particularly satisfactory. There is no real maximum thickness for the gold layer, but economy would dictate that it only be thick enough to insure that it be infrared radiation reflective. In a preferred embodiment, illustrated by FIG. 5, a thin strike or chromium layer 24 is applied over the potassium silicate layer 22 prior to deposition of the gold in order to improve adhesion of the gold to the bottom of the crucible. The chromium layer may be deposited by vacuum deposition, and should have a maximum thickness of about 200 Angstrom Units.

In order to demonstrate the advantages of the present invention, three crucibles of the type illustrated by FIG. 1A were prepared and compared with regard to the radiation emitted from the back surface of the crucibles during vacuum deposition conditions. Each of the crucibles is 2 X 5 inches in size and made of 304 stainless steel 0.025 inches thick. Each crucible was used to vacuum evaporate a photoconductive selenium-arsenic alloy for about 5 hours at deposition temperatures in the range of about 300 to 320C. All three crucibles have a 4500 Angstrom Unit protective top layer of SiO.

The first crucible comprised a stainless steel open boat crucible of the configuration illustrated in FIG. 1 which contained no protective bottom or back coatings of any type. The second crucible comprised a polished and silicated backing, but no gold layer. The third crucible comprised a polished silicated crucible containing the gold backing of the configuration illustrated in FIG. 3. The potassium silicate layer for the second and third crucibles is about one-half micron thick. The gold outer layer for the third crucible is about 800 Angstrom Units thick and a chromium layer about 200 Angstrom Units thick is disposed between the silicate and gold layers. These crucibles were heated by electrical resistance to a boat temperature of about 300C. A thermocouple is positioned about 1 inch from the back of each crucible in order to measure the temperature of the infrared radiation being emitted from the back surface of each crucible.

In order to obtain relative measurements of infrared radiation, it was necessary to construct a special pick up apparatus. An aluminum plate 0.080 inches thick by 18 inches long by 2%: inches wide was black anodized on one side and left shiny on the other. By facing the black side towards the bottom of the crucibles to be tested, infrared radiation would be absorbed into the aluminum plate. The side facing away from the crucibles is a shiny surface, and radiates very little and loses very little of the absorbed infrared radiation energy. A thermocouple was embedded in the bottom of the aluminum plate and attached to a multi-point recorder where the temperatures were recorded and used to generate the graphs of FIG. 4.

The temperature of radiation emitted from the back of each crucible is plotted in FIG. 4 for each of the three crucibles described above. It can be seen that after about 30 minutes of heating, the radiation emitted from the uncoated crucible is about 200C, while the polished and silicated crucible emitted radiation at about 150C. The silicated crucible containing the gold backing of the present invention, however, emitted radiation only at C or substantially less than half the radiation emitted from the uncoated crucible.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be within the scope of this invention.

What is claimed is:

1. A metal crucible apparatus having a highly infrared radiating upper surface which includes at least one material selected from the group consisting of tantalum, molybdenium, columbium, stainless steel and SiO which includes a cavity suitable for holding a source of evaporant material, said crucible having a bottom or underside which contains a thin homogeneous reflecting coating of gold, with the gold coating being confined to the bottom of said crucible.

2. The apparatus of claim 1 in which the underside contains a coating of potassium silicate having a thin overcoating of gold.

3. An open boat, stainless steel crucible having a highly infrared radiating upper surface which comprises a shallow dish portion suitable for holding a charge, and an underside containing a thin homogeneous outer layer of gold.

4. The apparatus of claim 3 in which the gold layer is at least 600 Angstroms thick. 7

5. An open boat, stainless steel crucible having at its upper surface a shallow dish portion suitable for holding a charge, an underside containing a silicate layer, a thin chromium layer overlaying the silicate layer, and a substantially continuous layer of gold overlaying the chromium layer.

6. The crucible of claim 5 in which the gold layer is at least 600 Angstrom Units in thickness.

7. A metal crucible apparatus having a highly infrared radiating upper surface which includes at least one material selected from the group consisting of tantalum, molybdenium, columbium, and stainless steel which includes a cavity suitable for holding a source of evaporant material, said crucible having a bottom or underside which contains a thin homogeneous reflectchromium layer overlaying the silicate layer, and a substantially continuous layer of gold overlaying the chromium layer.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3, 901, 647

DATED August 26, 1975 INVENTOR(S) Lloyd A. Relyea It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 28 delete "red".

Signed and Sealed this A ttes t:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ufPatenIs and Trademarks 

1. A METAL CRUCIBLE APPARATUS HAVING A HIGHLY INFRARED RADI
 1. A METAL CRUCIBLE APPARATUS HAVING A HIGHLY INFRARED RADILECTED FROM THE GROUP CONSISTING OF TANTALUM, MOLYBDENIUM, COLUMBIUM, STAINLESS STEEL AND SIO WHICH INCLUDES ACAVITY SUITABLE FOR HOLDING A SOURCE OF EVAPORANT MATERIAL, SAID CRUCIBLE HAVING A BOTTOM OR UNDERSIDE WHICH CONTAINS A THIN HOMOGENEOUS REFLCTING COATING OF GOLD, WITH GOLD COATING BEING CONFINED TO THE BOTTOM OF SAID CRUCIBLE.
 2. The apparatus of claim 1 in which the underside contains a coating of potassium silicate having a thin overcoating of gold.
 3. An open boat, stainless steel crucible having a highly infrared radiating upper surface which comprises a shallow dish portion suitable for holding a charge, and an underside containing a thin homogeneous outer layer of gold.
 4. The apparatus of claim 3 in which the gold layer is at least 600 Angstroms thick.
 5. An open boat, stainless steel crucible having at its upper surface a shallow dish portion suitable for holding a charge, an underside containing a silicate layer, a thin chromium layer overlaying the silicate layer, and a substantially continuous layer of gold overlaying the chromium layer.
 6. The crucible of claim 5 in which the gold layer is at least 600 Angstrom Units in thickness.
 7. A metal crucible apparatus having a highly infrared radiating upper surface which includes at least one material selected from the group consisting of tantalum, molybdenium, columbium, and stainless steel which includes a cavity suitable for holding a source of evaporant material, said crucible having a bottom or underside which contains a thin homogeneous reflecting coating of gold, with the gold coating being confined to the bottom of said crucible.
 8. A METAL CRUCIBLE APPARATUS HAVING AN UPPER SURFACE, A CAVITY SUITABLE FOR HOLDING A SOURCE OF EVAPORANT MATERIAL, AN 